(1) Field of the Invention
The present invention relates to a process for the production of a titanium (commercially pure titanium) or titanium alloy material having an excellent fatigue strength and workability. More particularly, the present invention relates to a process for the production of a titanium or titanium alloy material having a fine equiaxial microstructure.
(2) Description of the Related Art
Since titanium and titanium alloy materials have a high specific strength (high strength-to-density ratio) and an excellent corrosion resistance, they are used for the production of airplane parts and in many other materials, and the uses of these materials have been expanded. The reason why a titanium material, an .alpha.-type titanium alloy material, and an (.alpha.+.beta.)-type titanium alloy material are in such demand is that they have an excellent strength and ductility. The requirements for the properties are very severe in respective fields, and especially in the field of airplane parts to be used in the environment where a repeated stress is applied, not only an excellent workability but also an strong fatigue characteristic is required, and severe quality standards (such as seen in AMS 4967) have been stipulated. To satisfy these requirements, the microstructure of the material must have a sufficiently fine .alpha.-phase equiaxial grains.
In a commercially pure titanium material, however, since the impurity components are limited, it is impossible to realize a uniform and fine microstructure by conventional processing and heat treatments, although an equiaxial microstructure can be produced.
For the .alpha.-type titanium alloy material and (.alpha.+.beta.)-type titanium alloy material, shaped materials such as sheets, wires, tubes and rods to be used in the above-mentioned fields are generally prepared by the combination of a hot rolling and a heat treatment, but the conventional technique is defective in that, at the hot rolling step, the range of temperatures suitable for (1) maintaining a workability good enough to obtain a material shape having a high precision and (2) producing an equiaxial microstructure in the material, is very narrow.
Furthermore, in this temperature range, the microstructure of the material is easily changed by a change of the temperature, even by a slight rise of the temperature, and crystal grains grow and the microstructure after the treatment is often uneven. Moreover, a problem arises in that the microstructure formed by the hot working is little changed by a subsequent heat treatment.
Under this background, the following processes for obtaining .alpha.-type and (.alpha.+.beta.)-type titanium alloy materials having an equiaxial microstructure have been proposed.
(1) Japanese Examined Patent Publication No. 63-4914 discloses a process in which heating and working are repeated in a specific narrow temperature range. This process, however, is defective in that the microstructure cannot be made sufficiently fine and uniform and the attained equiaxiality is still unsatisfactory, and the productivity is low and the manufacturing cost high.
(2) Japanese Examined Patent Publication No. 63-4908 discloses a process in which a hot-rolled material is heated in a specific temperature range of the single .beta.-phase and is heat-treated. This process is defective, however, in that a microstructure which is sufficiently uniform and fine cannot be obtained and the attained equiaxiality is unsatisfactory.
A technique of improving the workability or microstructure of titanium alloy by adding hydrogen as a temporary alloy element (hydrogenation) is known, and the following processes utilizing this technique are known.
(3) U.S. Pat. No. 2,892,742 (June 30, 1958) to U. Zwicker et al discloses a process in which hydrogenating 0.05 to 1 wt. % of hydrogen in an .alpha.-type titanium alloy containing at least 6 wt. % of Al to improve the hot workability, and finally, the material is dehydrogenated by heating in a high vacuum. This reference, however, does not mention the microstructure of the material.
(4) In W. R. Kerr et al, "Hydrogen as an Alloying Element in Titanium (Hydrovac)", Titanium '80, pages 2477 through 2486, it is taught that if an (.alpha.+.beta.)-type alloy, Ti-6Al-4V, is hydrogenated, the .beta. transformation temperature is lowered and the hot workability is improved, and a fine microstructure is obtained. The hot working, however, is carried out at a reduction not higher than 60% by forging, and this forging is performed by the slow speed ram motion system in which the ram speed of the press is as low as 1.27.times.10.sup.-3. Accordingly, this process cannot be applied on an industrial scale.
(5) In N. C. Birla et al, "Anisotropy Control through the Use of Hydrogen in Ti-6Al-4V Alloy", Transactions of the Indian Institute of Metals, Vol. 37, No. 5, October 1984, pages 631 through 635, it is taught that if an (.alpha.+.beta.)-type titanium alloy, Ti-6Al-4V, is hydrogenated and hot-rolled, the anisotropy of the tensile properties is improved. According to the taught process, however, a hydrogenated plate is subjected to homogenization at 990.degree. C. for 2 hours, and is then rolled at 730.degree. C. at a total reduction ratio of 50% in several passes of 10% reduction each with a homogenization treatment of 10 minutes after each reduction, and this process cannot be applied on an industrial scale.
A material having a sufficiently fine and equiaxed microstructure cannot be obtained by these conventional techniques, and titanium and titanium alloy materials having an excellent fatigue strength and workability cannot be stably prepared on an industrial scale by these conventional techniques.